Method of removing noise from seismic signals



W. W. GRANNEMANN METHOD OF REMOVING NOISE FROM SEISMIC SIGNALS Filed Nov. 29, 1955 2 Sheets-Sheet 1 1/ 1a I REcoRoER REPRODUCER 14 /5 1s NOTCH NOTCH NOTCH FILTER FILTER FILTER I F l 24 I l L ossER l l I II I z I I 3/ 2s I l I I PHASE cLIPPER I coRREcTIoN I 1 32 26 l DFC l l DFC FILTER FILTER I l 33 27 I II AMPLIFIER AND AMPL'F'ER PI-IAsE INVERTER 28 l v I v ADDER l l L. 35 /36 37 SQUARE AND sQuARE AND SQUARE AND NTEGRATE INTEGRATE INTEGRATE as a9 40 PHASE PHASE 1 PHASE coRREcTIoN CORRECTION CORRECTION ADDER INVENTOR WAYNE w. GRANNEMANN 42 F 1 RECORD arch 1 W. W. GRANNEMANN METHOD OF REMOVING NOISE FROM SEISMIC SIGNALS Filed Nov. 29, 1955 2 Sheets-Sheet 2 1/ 12 13 RECORDER I REPRODUCER F I NOTCH f" -NOTCH NOTCH FILTER FILTER FILTER 20 2/ 22 DIFFERENTIATOR/ DlFFERENTlATOR DIFFERENTIATOR 44 J V i I 24 I l LOSSER W l V /46 25 I I AUTOMATIC PHASE ADJUST CL'PPER 43 45 49 I 26 i W DFC 1 PHASE DF c i COMPARATDR F'LTER l l l l 48 47 M v l I AMPLITUDE SUBTRACT T A. COMPARATOR II I AND ADJUST l I V I E l L II I as 36 :7 v m I I sOuAREAND I sOuAR AND SQUARE AND INT ORAT INTEORATE INTEGRATE INVENTOR REcoRD ADDER WAYNE w. GPANNEMANN BY I F| G 2 M,

TORN EYS United States Fat-cut O F METHOD OF REMOVING NOISE FROM SEISMIC SIGNALS Wayne W. Grannemann, Fullerton, Califi, assignor to Cahfornia Research Corporation, San Francisco, Calif., a corporation of Delaware Application November 29, 1955, Serial No. 549,593

9 Claims. (Cl. 3411-45) This invention relates in general to seismic prospecting and is directed more particularly to the elimination of noise from seismic detector signals.

In the art of seismic prospecting, a seismic impulse is generated somewhere within the earths crust and the resultant earth movement at a plurality of points more or less spaced from the point of origin of the impulse is recorded by means of seismic wave detectors. The original impulse sets up an elastic wave which is transmitted through the earth, and this wave, in general, will comprise a damped wave train. Any discontinuity or variation of structure within the earth will reflect and/ or refract or diffract this wave train or a portion thereof so that the record made at the receiving point will comprise a number of arriving waves, each differing from the others in time of arrival or magnitude, or both.

The reflected or refracted wave energy whose measure is desired is generally very small and requires instruments of high sensitivity and high magnification for satisfactory detection. Furthermore, the earths surface is often in motion, due to wind and wave action, traific, pulsations from heavy machinery and to unwanted forms of wave motion generated by and radiating from the explosion. When this extraneous energy is of the order of magnitude of the desired signal, interference results which diminishes the reliability of the data, and when, as is often the case, the extraneous energy is greater in magnitude than the desired signal, it may be impossible to recognize the reflected or refracted Waves with any degree of certainty. Thus, the usual seismic detector trace comprises a more or less continuous noise portion, with signal portions, representing reflections of interest, superposed thereon.

In general, a seismic reflection in a seismic detector trace can be approximated by cycle sine wave. Thus, if the seismic record can be analyzed by eliminating the portions thereof having a duration of more than and less than 1 /2 cycles, such action should eliminate all noise having a duration of either more than or less than 1 /2 cycles without substantially aifecting the reflections. After such action, the remaining portion of the seismic detector trace should comprise the 1 /2 cycle approximations of seismic reflections and any noise having a duration of approximately 1 cycles. Although the above discussion is based on a pulse duration of 1 cycles, it will be understood that other pulse durations, such as 2 cycles, may be utilized where changes in subsurface conditions or changes in the seismic detectors or amplifiers make such a pulse length more suitable. That is, the 1 /2 cycle pulse length is an average value which may be adjusted for optimum results.

Broadly the present invention contemplates methods I and apparatus for analyzing seismic detector traces in which the seismic reflection or signal portion thereof is approximated as substantially a 1 /2 cycle sine wave. The trace is first passed through a network which substantially' equalizes the amplitudes except for signal and a one and one-half (1 /2) noise portions of 1% cycles or less. The trace is'then ice passed through a network, which substantially eliminates the signal portions and noise portions of longer than 1 /2 cycles duration, so that'only the 1 /2 cycle signal portions and the noise portions of a duration of 1 /2 cycles and less remain. These remaining portions are then passed through a network which eliminates portions of less than 1% cycles duration, so that only the 1 /2 cycle portions remain. These 1 /2 cycle portions may contain bothsignal and noise, although there may be no 1 /2 cycle 'noisepresent in the particular trace, in which case only 1 /2 cycle signal portions remain.

More particularly, in the present invention the seismic detector trace is first passed through one or more narrow pass band filters to confine the trace to a narrow frequency band or bands. The output from the narrow pass band filter is then supplied to a dominant frequency eliminating network which includes a losser feedback network for controlling the amplitude of the signal passing therethrough. The losser feedback network has a time constant such that the first 1 /2 cycles of a portion of the signal of increasing amplitude pass through the losser without efiect, but portions subsequent to the first 1 /2 cycles are strongly attenuated. The output from the losser network is then supplied to' a network which removes the noise portion of greater than 1 /2 cycles so that the seismic reflection of 1 /2 cycles and noise of 1 /2 cycles and less remain. This signal is then supplied to a suitable net- Work, such as a squaring and integrating network, where portions having aduration of less than 1% cycles are eliminated. The output from this last-named network then would ideally comprise the 1% cycle signal portions 1% cycle portions, if present, removed therefrom.

In additional form of the present invention, the seismic detector trace may be differentiated prior to passage through the dominant frequency cancellation network to emphasize reflections or signal portions which appear in the original trace as phase variations. As a further refinement of the present invention, the phase and amplitude of the signal in the dominant frequency cancellation network may be automatically regulated to provide a more exact cancellation of the undesired noise portion.

The nature and objects of the present invention will be further apparent from the following description when read in connection withthe accompanying drawings, in which:

Fig. l diagrammatically illustrates one the present invention, and

Fig. 2 diagrammatically illustrates an alternate embodiment of the invention in which automatic phase and amplitude regulation is utilized in the dominant frequency cancellation network.

Referring to Fig. 1 by character of reference, numeral 11 designates a seismic wave detector for producing a seismic trace having an amplitude corresponding to movement of the earth in response to-a seismic disturbance. The output signal from detector 11 is supplied through an amplifier 12 to a suitable reproducible recording device generally designated as 13. Recording device 13 may be of any suitable known type, such as a photographic or magnetic medium, on which the seismic detector traces may be recorded and reproduced repeatedly and at will. Although only one seismic detector and its associated amplifying channel have been illustrated, it will be obvious to those skilled in the art that in practice a large number of such detectors are normally utilized for eachseismic disturbance and that such detectors may be arranged in any suitable configuration.

After the seismic detector trace is recorded on device 13 through amplifier 12, the trace is reproduced from device 13 and supplied to a plurality of narrow band pass filters connected in parallel,

embodiment of l atented Mar. 11, 1958' In the embodiment illusasses-he trated in Fig. 1', three such filters, 1'4, 1'5 and'16', are shown, although it will be understood that the number i. SE91}. filte she ar esl. nz denqhs s sehpsn the. m r o difisreht trea e y hand nte es hi. t e smi dhteq ns s ala Th h tpms tq nlte s ;..15; 5 4. 6 at suppliesl en ate domi ant. t siys szy 11 pon n e imina i n: Q1". han ellat a he axk t The. e imiz i ian. nq wq h. assoc a e w th. i t r 15: qqmn he emc ts hclps diwi hin de the, lqtte ehcla hre hsl s dl by he ershse charas e fi h qt uch etwor s. r d en lt r s. pif ie ah dentica nd ac d.- ly. .3, th .w h. 1..9. T S1 9.fi I$- 1l15 s. Shown in detail- The. qa eellatisih ne wb h a h i sa th filte s. .4 and. e. haw s he atisa ly @517: and respes iyely t ri g. IIHQEITSIQQQ. ha hes network re l ls ahtia ily dsnt sa t h t y h.

Dom na t frequenc ahsel atisn e w .8, ncludes. a ess r. net ork awh sh. he s gna frhnfi t r 1.51sv ppl ed. and. hishaqtsasa e tqm t s g l a nta n m amph f hesgha gas ng. here; r h h. thin, redete mined imits, The out 19 1. te q nz sss a. a e. tra n. iab mpl t e avin a. treah s ying. h n he P ss. h a f fi te That sih wa et a n r mfilter may be com deredl compr se. am ss. h ti hq ns ant n. slow r ng amp itude. w th. ihtet p rsed; n ti n Q i ab up ly creasin m litu e. eprese t n igha radia s. urf r.- sed; h. he ia se or 9h.-.. l-a se jhe w kz fii ha a. e n tahts ch hat the. Po ti n .ofiqqnst hto q y y ry n mp itude. are esiua izei n. amplit de ht hen e amiz ithd f; he SiEQEiI HPP EQ 9 l ss r n reases. b up ly. nn iqximats th first cy es of his. 2 3 cs. of. inst .asing. mp tude ass hran h. osse netw rk h tanti l yun han ed. n m litude. haters. lesser 2. acts to reduce' the amplitude. Thus, the output from ser netwq lfi. cshz t ses. aw y t hih q substant a ly n tant m tude w th. ntermit nt. 2. yc xtiqhs. c eased. am litude. nte ersed her e. Th fir t plus.

cyc es. 3. the. s al Part san. o ha s in t me q hs slsh e l sse 16?" h. hi igna no tiqm re. hu missed by qt sli.

h q tnut tram. Qsse ne w k. is h n. washed in. common to. a Pair q pata lel. hr nhh hetWQr Qs-I 29. an h Q .l i 1. i Q QQmRli5Q c i er. ha ing, n OIIIRIJP. qhhsctsd. to a. sn ci h h ter 2 Whose uinn? in t n. s. con ec ed. o. am i st a d. phase nve e network 27. The output of amplifier and phase inverter tyyqtk. .7. su nhsd s; an. adder net qh s. it, C pe 25 acts to produce a constant amplitude square wave qse axi rea in Pa nt or frequency qqrie p n o e. a q ns ps. minis r. re e y Q f h n l. f Q P. ss r. 2.4-. Th s uare. W Q chn taht at m ih de rQ h clipper 25 is supplied to filter 26'which removes the harhiqs Q th s uare ave and. Phd i -I aw: y g. a. qns ntfi mpl tud ..a" r.e uq y de ermine by he. re uen y qffths s n l. rqm' se 24-. Th s soida a r ih s u ii... t q n w k .7 w e it s. mpl fied. h tt d tihase, and. pp ied 9. dde network 2 8 The Q her rahsh 9 h na l sl. ne w k campu e a phase correction circuit 371" which serves to vary the P as Q h s nal. gas n h re ht h h i a qq dan e th th Pha e .chahges hit a e. ntt dilh din he. o he n h. s n l n. passing hrqush dia er work 25;. Phase q ec et ork. 31 m h s be a simplafilte twqrk h y t e. same s es s hsmiss pn, @hara sr s: i s. as Ql Pnsr'ZSJ. Th put 59. 2; ph se ane h h mark-3 .15 sshpp ie a identical to filter 26, v a inth hmiishanlanplifier nns-Qt aste twork 2& Netw r thts t e to. dd. he t osigna supplied, thereto, and: since. the. signal: supplied: from net.- work27 has. been. invented; in phase, thesignal'fromnet-.

. along the entire trace. and also. having abrupt amplitude variations of a few cycles d-uizat-ion, corresponding to a na nqr n sup rpos v us. times on h noi e, portion." In passing through losser 24,; the relatively s y changing amplitude a iatiqhs re equali ed n amplitude by the gain control action of network 24. However, the time constant of'losser' network 24- is such that the first 1 /2 cycles oi the relatively abruptly increasing signal portionwill pass through losser network 24 substantially unaltered in amplitude before losser network 24 responds to again equalize the wave train amplitude The output f r om"losser 24 would thus comprising a noise portion having an. a I stantially constant amplitude withsignalportions of creased amplitude and 'ofsubstantially HZ; cyol'es durationinterspersed therein. i i

In clipper 25,- this hypothetical wave. train. would b converted into a square wave having-a constant amplitude and a frequency determined by the frequency oi the input wavetrain. Thus, clipper 25fwould eifective ly eliminate the 1 cycle signal aha noise portion from the wave train supplied thereto. Filter 26. converts constant} amplitude square wave into a substantially sinusoidal wave train ofthe same frequency and of constant amplitude. When the two signals are combined in adder network 28, the constantv amplitude signal from network 27 canoe 1st its corresponding-portion in the wave train from amplifr fier-33. The difierience between the two wave trains sup;

plied to adder'2 8 is the presence in the wave train from amplifier; 33o]? the portions of- 11/; cycles and less. Thus, theneise portion of greater than 1 /2 cycles duration is effectively cancelled so that the output of adder 28 comprises the l /zcycle signal and noise portion plus a noise portion having a duration ofi less than 1% cycles:

The output from the dominant frequency'elimination network isthen supplied to a device for eliminating the noiseportion having a. duration of less than 1 /2 cycles; Such a device may be a squaring and integrating network 3&which actsto average the signal supplied thereto and eifectively eliminates the portions oi short duration. Sit nilarsquaringgand integrating networks 35 and 37 maybe providecl'fop dominant frequency elimination networks 17 and-19,- respectively. The outputs from networks 35; 36 and 3 7; then, ideally comprise only the 1 /2 cycle signal and noise portions which have frequencies lying within the pass bands of the respective filters 1'4, 15 and 16; Theoutputs from the squaring and integrating networks 35, 36 and 37- may then, ifdesired, be passed throughaplurality of phase correction networks 38; 39 and 40 foreliminating" any undesired phase distortion introduced: in the operation. The outputs from the phase correction networksare supplied jointly to an adder or summing network 41- where bined to producea resultant signal representing the sum of the. 1% cycle signal and noise} portions from the difierentparallel networks -T1 '1is resultant supplied to a suitablereeorder 42 fo1i'analysis.

The embodiment of- Fig. 1 isparticularly-well adapted fort-usewhere thereflections of'interest are superposed on the. neisetportionas amplitude changes and may-readilybe-detectedi afteu subtraction of thenoise portion. As mentioned earlier, however, the reflections of interest may sometimes appear. onthe. detector trace only as phasechanges therein. without any substantial elfecton the amplitude 'of the trace. Such phase changes are difiicultto detect, particularly, where. the total traceis substan-.. tially. a sinusoidal wave. train.

The embodiment illustrated in Fig. 2" may be utilized to convert the'se phase changes into amplitude changes be w ys t n. en e op Qfi' hthe different signals are com- 1 output signal is "prior to subtraction of the noise energy from the trace. Additionally, the embodiment of Fig. 2 provides for automatic regulation of the amplitude and the phase of the signals produced in the dominant frequency cancellation networks to produce moreexact cancellation of the noise. In Fig. 2, seismic detector 11 produces an output signal which is supplied through amplifier 12 to recorder/reproducer 13, and the output from recorder/reproducer 13 is supplied in parallel to notch filters 14, 15 and 16, as before. The outputs from filters 14, 15 and 16 are supplied to a plurality of suitable differentiating networks 20, 21 and 22 which serve to produce output signals proportional to the first derivative of the traces supplied thereto. Variations in the frequency of the input signals to differentiating networks 20, 21 and 22 are thus reflected as variations in the amplitudes of the differentiated output signals thereof. This increases the possibility of detecting the presence of reflections of interest in the resultant traces since, in general, a variation in amplitude is easier to detect than a variation in frequency, particularly in a wave train which is nearly sinusoidal over a relatively long duration.

The output from difierentiator 21 is supplied to a dominant-frequency elimination network which comprises the elements in the dotted enclosure 44. Similar dominantfrequency elimination networks 43 and 45 are provided for differentiating networks 20 and 22, respectively. Network 44 includes a losser 24 which functions in a manner identical to that described in connection with losser 24 in Fig. 1. The output from losser 24 is supplied to a pair of parallel branch networks. One branch includes an automatic phase adjusting network 46 whose output signal is supplied to clipper 25. The output from clipper 25 is supplied through filter 26 to an automatic amplitude comparing and adjusting network 47. The output from comparator 47 is supplied as one input to a subtracting network 48. The other input to subtracting network 48 is supplied from losser 24.

A phase comparator network 49 receives one input signal corresponding to the output signal of amplitude comparator 47 and receives another input signal corresponding to the output signal from losser 24. Phase comparator 49 acts to compare the phases of these two input signals and supplya control signal to automatic phase adjusting network 46 to adjust the phase of the signal passing therethrough to match the phase of the signal supplied from losser 24. The time constant of networks 46 and 49 is such that they act on the average phase averaged over a suitable time interval. That is, they have a time constant such that they do not appreciably affect the portions of 1 /2 cycles and act only on the portions exceeding 1%. cycles. 7

Amplitude comparator network 47 compares the amplitude of the signal from filter 26 with that of the signal from losser 24 and adjusts the amplitude of the signal from filter 26 to substantially equalize the amplitudes of the input signals to subtracting network 48. Amplitude comparing and adjusting network 47 has a time constant such that it does not appreciably affect the 1V2 cycle portions and acts only on those portions exceeding 1 /2 cycles. The phase and amplitude of the signals in cancellation network 44 are thus automatically regulated during the operation so that the cancellation action of the network will bemost effective. The dominant-frequency cancellation networks of Fig. 2 thus operate in a manner similar to that described above in connection with Fig. 1 to eliminate the noise portions of greater than 1% cycles from the 1 /2 cycle signal and noise portions.

The output from subtracting network 43 is supplied to squaring and integrating network 36. The outputs from dominant-frequencycancellation networks 43 and 45 are similarly supplied to squaring and integrating networks 35 and 37 respectively; Squaring and integrating networks 35, 36 and 37 operate in a manner similar to that described above in connection with Fig. 1 to smooth or eliminate the portions of less than 1 /2 cycles duration. The outputs from networks 35, 36 and 37 are supplied to a summation network 41. These signals are combined in network 41 to produce a resultant signal which is supplied to recorder 42.

The embodiment illustrated in Fig. 2 has the advantage with respect to the embodiment of Fig. 1 that one output from losser 24 is sup-plied directly to one input of subtracting network 48 so that no phase distortion is introduced into this signal. The embodiment of Fig. 2 has the additional advantage that the use of automatic phase and amplitude regulation to a large extent eliminates the need for manual adjustment of these quantities, thus increasing the accuracy and the speed of operation of the system. The embodiment illustrated in Fig. 1, on the other hand, has the advantage thatthe output circuit from losser 24 is substantially perfectly balanced through both of the branch networks, thus reducing the possibility of asymmetrical phase distortion in the two branches.

Although but a few illustrative embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. Apparatus for analyzing a seismic detector trace to emphasize signal portions therein which are superposed on a substantially continuous noise portion, said signal portions having increased amplitude with respect to said noise portion, comprising a filter network having a narrow pass band for confining said trace to a band of frequencies within said pass band, a variable losser net work connected to the output of said filter network, said losser network having a time constant such that approximately the first one and one-half cycles of portions of said: trace of increasing amplitude pass through said losser network without any substantial change in amplitude and; the other portions of said trace are substantially equal-- ized in amplitude, means connected, to said losser net- 'work for producing a substantially sinusoidal signal hav-- ing a substantially constant amplitude and having a frequency determined by the frequency of said noise portion, and means for subtracting said sinusoidal signal from the: output signal of said losser network to produce a resultant signal comprising a series of one and one-half cycle components corresponding to said signal portions.

2. Apparatus for analyzing a seismic detector trace to emphasize signal portions therein which. are superposed. on a substantially continuous noise portion, said. signal; portions having increased amplitude with respect to said;

' noise portion, comprising a filter network having a nar row pass bandfor confining said trace to a band of fre quencies within said pass band, a variable losser network. connected to the output of said filter network, said losser network having a time constant such that approximately: the first one and one-half cycles of portions of said trace; of increasing amplitude pass through said losser network without any substantial change in amplitude and the otherportions of said trace are substantially equalized in am-'- plitude, a clipping network connected to the output of said losser network for producing a square wave signal having axis crossing points corresponding to the axis. crossing points of the output signal from said losser net-- work, a smoothing filter for converting said square wave signal into a substantially sinusoidal signal having a sub-- stantially constant amplitude and having a frequency determinad by the frequency of said noise portion, andi means for subtracting said sinusoidal signal from the output signal of said losser network to produce a re-- I 3. Apparatus for analyzing a seismic detector. trace to emphasize signal portions therein which-are superposed ona substantially continuous noise portion; said-"signal portions having increased amplitude with respect tosaid noise portion, comprising a filter network having a narrow pass band for confiningsaid trace to, a'ba-nd of frequencies within said pass band, a variable losser network connected to the output: of; said filter net-work, said losser network 'having'a timeconstant such that approximately the first one and one-half cycles of portions of said trace of increasing amplitude pass through saidlosser network without any substantial change inamplitudeand the other portions o f sa id trace are substantially equalized; in amplitude, a clipping network connected to the output of said losser network for producing asquare wavesignal having axis crossing points corresponding to the axis crossing points of tlae output signal fromsaid losser network, asmoothing filter for converting said square wave signal into a substantially v sinusoidal signal having a substantially constant amplitude and having a frequency determined by the frequencyof said noise portion," means for subtracting; said sinusoidal signal from the output s i'gnalof said lossepnetwork to produce a resultant signal comprising a-series of one andone-half cycle components correspondingto said signal portions, and means for averagingsaid resultant signal to remove therefrom portions having a duration of less than one and one-half cycles. l u

4*. Apparatus for-analyzing aseismic detectortrace to emphasize signal portions therein whichare: superposed on asubstantia-llycontinuous noise portion, slaid signal portions having increased amplitude withrespect-f 'd noise portion, comprising a variable losser network; said losser network having a "time constant such that: approximately the first one-"and one-half cycles of portions oi: said trace of increasing amplitude pass mrougrr'saia" losser network without any substantial change than tude and the other portions of said trade are snbstang tially equalized in amplitude, a clipping nerwiqngpoa nected to the output of said losser ne work-- for'produe ing a square wavesignal having axis crossing points corresponding to the axis crossing points of the output signal; from-saidl'ossernetwork, a smoothing filter for converting said square wave signal into a substantially sinusoidal signal, having -a substantially constant 'amplia tude and havinga frequency determijnedby the frequency of a d i t; new and m n fb g uh ra 198, aid Sinusoidal signal om t e Q HP I ig al "s idllqsssr. work to M951 1??? rssult nt nal, am i na s r of one and cl romn netts p i sn n'qina to ne-hali dsi a r r qas v d pa fat o n l z ng, a s ism new trace is has z nal BQI iQI ther in WhJQhQI? uparnq ed' on a antiall c ntinu s .nQi por ion at si n l P t o having q sasstli m l tu s lv hs ps tn is noise o io omrrisins ter t orkv ha ing ana w Pa a d Qr c n n g ai ress, ome s; of; qq r ssr it in aidrassfhan; vd fier m in me a for i er ntiatin ai ltered ra e, a: peta s, r rk n es ed 91 1? 9mm; of sa d' d ficrentia msai ser n twork hav ng. a time w nt sushi that n wx mate x he first qn rani Queih. P tions f ai di fe a atsd tra otai asi l rude pa ou h sai osser. st Qrl v 7.0. any; b stantial. cha n na du nd, he o her part als t said ditferentiated trac a. ns b t m al r fa ua i u e. cl p naa tw ,rk, on egtedtq t e Q put of a as e i artw rk .01; rm usinsa quarewa esia: nal havingaxis crossing po ts corresponding tofthe' axis crossing pointsof theou ut signalf fi iljlosser. net; work, a smoothing filter forconverting said, squarewave signalinto a substantially sinusoidal; signal having a substantially constant amplitude andliavirig' a frequency; determined by! the; frequency-L o= said n'ois6 portion; and means for subtracting-saidsinusoidalsignal-from the-output signal of said losser network to produce a resultant 8 signal comprising a series of one and one-half cycle components corresponding to said signal portions.

6. Apparatusfor analyzing a seismic detector trace to emphasize signal portions therein which are superposed on a substantially continuous noise portion, said signal portions having increased amplitude with respect to said noise portion, comprising a filter network having a narrow pass band for confining said trace to a band of frequencies within said pass band, differentiating means connected to said filtering network for dilferentiating said filtered trace, a variable losser network connected to the output of said dilferentiating means, said losser network having a time constant such that approximately the first one and one-half cycles of portions of saiddifi'erentiated trace-of increasingamplitude passesthrough said losser network without any substantial change in amplitude and the other portions of said differentiated trace are substantially equalized in amplitude, aclipping network connected: to; the output of said losser network for producing a square wave signal having axis crossing points corresponding to the axis crossing points of the output signal from said losser network, a smoothing filter for converting said square wave signal into a substantially sinusoidal signal having a substantially constant amplitude and having a frequency determined by the frequency of saidj noise 'portion, means for subtracting-said sinusoidal signal from theoutput signal of' saidlosser network to-produce a resultant signal comprising a series of one and one-half cycle components corresponding to said signal portions, and means for averaging said resultant signalto remove therefrom portions-having a duration of less than one and one-half cycles.

7-. Apparatus-for analyzing a seismic detector trace to emphasize signal portionstherein which are superposed on a substantially continuous noise portion, said signal portions having increased amplitude with respect to; said able lossernetwork connected to' the output of "each of said filter networks, eachgof said losser networks having a time constant such thatapproximately thefirstone, and one-half cycles ofportions 'of said filtered trace of" increasing amplitudepass through said'losser networkwithout any substantial change amplitude and the other portions of said filteredtrace are substantially equalized in amplitude, a-clippingnetwork connected'tothe output of each of said losser networks for producing wave signal having"axis crossing points corresponding to the axis crossing points' of the'output-signal'from, said losser network, a plurality ofsmoothing filters for converting each of saidsquare wave signals into a'rsubstantially, sinusoidal signal having a substantially constant amplitude; and having a frequency determined by the frequency o fsaid noise portion, andgmeans tfor subtracting eachofsaid sinusoidal signals from the output signal,

able losser network connectedlto the output of each of said: filter-networks,f each of said-losser networks having a timeconstant such'thatapproximately the first one and one-lialieycles of portions ofsaid filtered' traceof increasingzarnplitud'e pass through said losser network withoutanysubstantial change in amplitude and the other portionsofsaidfiltered traceare substantially equalized in amplitude, a clipping network connected to the output a square of each of said losser networks for producing a square wave signal having axis crossing points corresponding to the axis crossing points of the output signal from said losser network, a plurality of smoothing filters for converting each of said square wave signals into a substantially sinusoidal signal having a substantially constant amplitude and having a' frequency determined by the frequency of .said noise portion, means for subtracting each of said sinusoidal signals from the output signal of the corresponding one of said losser networks to produce a plurality of resultant signals comprising a series of one and one-half cycle components corresponding to said signal portions, and means for averaging each of said resultant signals to remove therefrom portions having a duration of less than one and one-half cycles.

9. Apparatus for analyzing a seismic detector trace toemphasize signal portions therein which are superposed on a substantially continuous noise portion, said signal portions having increased amplitude with respect to said noise portion, comprising a plurality of filter networks having narrow pass bands for confining said trace to a frequcncy range determined by said pass bands, differentiating means connected to each'of said filtering networks for differentiating each of said filtered traces, a variable losser network connected to the output of each of said 25 differentiating means, each of said losser networkshaving a time constant such that approximately the first one and one-half cycles of portions of said diiferentiated trace of increasing amplitude pass through said losser network without any substantial change in amplitude and the other portions of said differentiated trace are substantially equalized in amplitude, a clipping network connected to the output of each of said losser networks for producing a square wave signal having axis crossing points corresponding to the axis crossing points of the output signal from each of said losser networks, a plurality of smoothing filters for converting each of said square wave signals into a substantially sinusoidal signal having a substantially constant amplitude and having a frequency determined by the frequency of said noise portion, means for subtracting each of said sinusoidal signals from the output signal of the corresponding one of said losser networks to produce a plurality of resultant signals comprising a series of one and one-half cycle components corresponding to said signal portions, and means for averaging each of said resultant signals to remove therefrom portions having a duration of less than one and one-half cycles.

References Cited in the file of this patent UNITED STATES PATENTS 

